Servocontrolled pressure limiting valves are widely used in the distribution of pressurized fluid whenever it is necessary to rapidly drain off relatively large quantities of fluid when the pressure exceeds a predetermined level. Such valves, also known as pilot-operated valves, essentially comprise a main piston and a pilot or servocontrol valve associated with the main piston, the servocontrol valve being a relatively small, directly acting pressure responsive valve. The smaller valve opens when the predetermined pressure is exceeded, thereby relieving one face of the main piston and, as a result of the fluid pressure differential, the main piston is moved in the opening direction. Although such servocontrolled pressure reducing valves are used in large numbers and operate reliably when properly maintained, the main piston tends to be quite heavy, particularly when the valve is a large size valve, so that a large mass must be accelerated at the moment of opening, and this has a detrimental effect on the opening time.
The weight of the main piston can be reduced if a cartridge type of pressure limiting valve is used. In such a valve, the main piston is constructed so as to have a hollow, cup shape which results in a considerable reduction in the mass of the piston. The advantage of this construction is particularly apparent when this valve is used as a single-acting pressure limiting valve wherein the feed side, i.e., the higher pressure side, is closed by the bottom of the cup-shaped main piston. In single-acting pressure limiting valves, this surface can be constructed in such a way that it is only slightly smaller than the total surface of the main piston which means that the valve immediately responds in response to only a small pressure reduction on the side of the main piston opposite the bottom. The smaller mass of the main piston and the lower pressure differential required to make the main piston operate ensures that the valve responds somewhat more rapidly than in the case of a solid piston. If such cartridge-type valves are used in fluid systems in which the pressure side alternates, so that a higher pressure can occur at either pipe connection, a second valve must be provided for the other pressure side because such a valve can only respond to one pressure side. This represents a relatively large expenditure.
Thus, solutions have been sought for the purpose of providing a single valve which can respond to high pressure at either side. In one such known structure, shown in Swiss Pat. No. 364,671, the cup-shaped main piston has a bottom surface to which is connected one pipe connection and, outside the bottom surface, the piston has a second annular surface associated with the second pipe connection. Because it is to operate as a servocontrolled valve, it is necessary to control the operation in such a way that the servocontrolled valve is always coupled to the pipe connection in which the higher pressure prevails. In the known valve, this is achieved by providing two check valves in the main piston, one of the check valves being connected in each case to one of the two pipe connections. Thus, the check valve which is at the higher pressure is open and the other is closed.
Although this arrangement solves the aformentioned problem of providing two pressure responsive sides with a single valve, it leads to the disadvantage that both the bottom surface and the annular surface of the main piston are smaller than the total surface of the piston. If, for example, the two partial surfaces are of the same size, the total surface of the piston is doubled. Since, however, the response of the main piston occurs as a result of the pressure on one of the partial surfaces, there must be a relatively large pressure differential before the main piston responds. However, because the diameter of the main piston corresponding to the two partial surfaces is larger than for the single-acting valve, the piston mass is noticeably increased, again slowing down the opening process.